It is known that certain materials can be added to iron to give preferential physical properties in alloying and forming steel. Thus, carbon, chromium, nickel, molybdenum and manganese have been commonly blended together with iron, in varying combinations and percentages, to increase tensile strength and hardness, add resistance against creep and fatigue, and improve resistance against high temperature degradation, oxidation and carbide formation.
For example, oxidation is a buildup at the surface of oxides, such as iron oxide on the conventional iron not having special additives. Certain oxide coatings, if nonporous and adherent, can reduce the rate of continued oxidation. Chromium, titanium and aluminum have very high rate of diffusion and when added to the iron become the first to oxidize. The grain boundaries and other defective regions (like dislocation lines) provide high diffusion paths for the oxidation. The oxides formed at the normal surface as well as these boundaries serve as a protective coating or barrier against continued rapid oxidation. The rate of oxidation in most metallic alloy systems containing chromium is determined largely by the rate of diffusion of the metallic species through the oxide layer. That is, the active oxidation is occurring at the oxide/oxygen (or air) interface. The diffusion rate of oxygen through the oxide layer is negligibly small.
After the formulation of the material has been settled and the metal made, certain post formation or preuse conditioning processes can be performed on these materials to further enhance their physical characteristics. Of concern, however, is the fact that most commonly, the improvement of one physical property (resistance against corrosion, for example) results in a reduction of another physical property (fatigue strength, for example). Thus, annealing improves mechanical strengths against fatigue and creep particularly at elevated temperatures and reduces stress buildups incidental to cold forming. However, annealing also generally reduces the basic tensile strength, hardness, and improves ductility at elevated temperatures. Nitriding and carburizing might be used for improving the surface hardness and resistance against wear.
"Super alloys" are also available, using nickel as a primary material with some of these same additives also as the primary materials, and minor percentages or only traces then of iron. Some examples of specific "super alloys" are:
(a) Inconel 625 having approximately 22-25% chromium, 61% nickel, 8-10% molybdenum, 3.5% niobium, 3.5% iron and traces of aluminum and titanium.
(b) Inconel 600 having approximately 15% chromium, 72% nickel, 8% iron, and traces of carbon, manganese, copper and silicon.
(c) Inconel 718 having approximately 17-22% chromium, 50-55% nickel, 4% niobium plus tantalum, 3% molybdenum, traces of manganese, silicon, copper, carbon, aluminum, cobalt and the balance of iron.
(d) Inconel 750 having approximately 15% chromium, 70% nickel, 7% iron, and traces of carbon, manganese, silicon, titanium and aluminum.
Each super alloy, by its nature, is intended to operate in areas of high demand where mere survival could be a success. The blends and proportions of the base materials and additives forming the various super alloys differ from one another in order to accomplish specific purposes for the alloy. Thus, large proportions of nickel add resistance against corrosion and increase hardness; increased percentages of chromium add durability and resistance against oxidation at high temperatures while yet having high tensile strength, increased molybdenum in ranges even up to 9% add strength and resistance against high temperature degradation and resistance against creep and fatigue; while increased percentages of niobium provide resistance against carbide formation.
The super alloys have melting points in the range of 1300.degree.-1350.degree. C. and high strengths at temperatures even above 650.degree.-825.degree. C. The super alloys also generally provide good resistance to fatigue and creep even at high temperatures and in corrosive atmospheres, and high resistance to oxidation that can be two to five times better than stainless steels. This would include high resistances against corrosion from marine or urban pollution, ammonia, hydrogen sulfide and sulfur dioxide for temperatures even in excess of 900.degree. C. Thus, any improvement in the performance of any of these super alloys with respect to fatigue strength, or in resistance against creep, or in resistance against oxidation, even if obtained singularly would represent a contribution to the art; but if obtained simultaneously, would be most significant.